1. Field of Technology
The present invention relates to alloys having low coefficient of thermal expansion. The present invention more particularly relates to alloys including iron and/or nickel and having low coefficient of thermal expansion, to methods of making such alloys, and to article of manufacture including such alloys.
2. Description of the Background of the Technology
The propensity to expand and contract on changes in temperature is a fundamental property of metals and alloys. A material's coefficient of thermal expansion variously refers to a change in length, area, or volume as a function of change in the temperature of the material. As used in the present disclosure, the coefficient of thermal expansion or “CTE” of a material refers to the coefficient of linear thermal expansion “α”, which satisfies the following equation I:ΔL/Lo=αΔT  (I)in which Lo is the original length of the object of interest (in the measured direction), ΔT is the temperature change to which the object is subjected, and ΔL is the change in the object's measured length that occurs with the indicated change in temperature, expressed in the same units as Lo. Thus, ΔL/Lo is a fractional change in length, and the CTE is a material property that may differ depending on, for example, the nature of the material. Equation I indicates that the fractional change in length is proportional to the change in temperature and, in fact, that relationship only holds for most materials over relatively small temperature ranges. Because a material's CTE may depend on the particular temperature range in which the property is evaluated, it is often necessary to specify the temperature or temperature range when reporting the CTE of a material. Conventional analytical methods for determining CTE include measurements utilizing a dilatometer or laser interferometry.
Certain applications require low CTE metals and alloys, i.e., metals and alloys experiencing relatively little change in linear dimension with changes in temperature. In many such applications, the necessity for low CTE materials derives from the need to maintain substantially fixed distances between critical elements of an apparatus, the requirement for an element of substantially invariable length, or the need to maintain structural soundness of an assemblage of parts subjected to large variations in temperature. Applications requiring high dimensional stability with variation in temperature include structures for sophisticated telescopes and other optical devices; certain telecommunications equipment components, including filters in mobile phone networks; shadow masks, frames and gun parts used in cathode ray tubes; tank membranes for liquified natural gas tankers; mold plates for aircraft structural composite material fabrication; and bimetallic strips for thermostats and other applications.
A particularly well-known family of low CTE alloys is the family of alloys including about 36 weight percent nickel and the remainder of iron and allowable levels of incidental impurities. This family of nickel-iron alloys is sometimes referred to generically as the “Invar” family of alloys and is referred to herein as the “36Ni/Fe” alloys. When the 36Ni/Fe alloy family was discovered in 1896, the alloys' unique property of low linear expansion over a wide temperature range was initially employed to produce bimetals used in safety cut-off devices for gas stoves and heaters. For his work on nickel-iron systems and the discovery of the 36Ni/Fe alloys, Charles Edouard Guillaume was awarded the Nobel Prize for Physics in 1920. As shown in the FIG. 1, which plots CTE as a function of nickel content in a nickel-iron binary alloy, the 36Ni/Fe alloy having exactly 36 weight percent nickel has the lowest CTE. In fact, an alloy of 36 weight percent nickel and 64 weight percent iron is generally regarded as having the lowest CTE among all alloys in the range from room temperature (about 20° C.) up to approximately 230° C. In general, 36Ni/Fe alloys are ductile and easily weldable, and have machining characteristics similar to austenitic stainless steel.
ASTM Designation F 1684-99, “standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications”, covers two common low thermal expansion 36Ni/Fe alloys, a “conventional” 36Ni/Fe alloy (designated UNS K93603) and a “free-machining” 36Ni/Fe alloy (designated UNS K93050). Each is nominally 36 weight percent nickel and 64 weight percent iron. Table 1 below provides the chemical requirements (in weight percentages) listed in ASTM F 1684 for these alloys. With one exception, these requirements relate to maximum allowable levels of various impurities, i.e., permissible deviation from the theoretical pure 36 weight percent nickel/64 weight percent iron alloy. The sole exception is with respect to selenium, which must be controlled to 0.15-0.30 weight percent in the free-machining alloy. Selenium is not measured (indicated as “NM”) in the conventional alloy.
ElementUNS K93603UNS K93050Iron, nominalremainderremainderNickel, nominal36   36   Cobalt, max0.500.50Manganese, max0.601.00Silicon, max0.400.35Carbon, max0.050.15Aluminum, max 0.10aNMMagnesium, max 0.10aNMZirconium, max 0.10aNMTitanium, max 0.10aNMChromium, max0.250.25SeleniumNM0.15-0.30Phosphorus, max  0.015b 0.020Sulfur, max  0.015b 0.020aThe total of aluminum, magnesium, zirconium and titanium cannot exceed 0.20 weight percent.bThe total of phosphorus and sulfur cannot exceed 0.025 weight percent.
36Ni/Fe alloys are commercially available from various sources including Allegheny Ludlum Corporation, Pittsburgh, Pa., which distributes an AL 36™ electrical alloy for cryogenic (UNS K93603) and bimetal and trimetal (UNS 93603) applications having the following typical weight percentage chemistry: 36.00 nickel, 0.008 carbon, 0.30 manganese, 0.001 sulfur, 0.15 silicon, less than 0.35 cobalt, less than 0.02 molybdenum, less than 0.03 aluminum and balance iron.
36Ni/Fe alloys have CTE in the room temperature range that is less than 1 part per million per degree Fahrenheit, represented as “<1×10−6° F.−1”. This may be compared with the CTE of carbon steel at about 6.3×10−6° F.−1 and of aluminum at about 12.4×10−6° F.−1. However, although the “Invar” name was coined to allude to the alloy family's “invariable” expansion, the CTE of 36Ni/Fe alloys does vary depending on variations in composition and the temperature range in which CTE is measured. For example, the CTE of one 36Ni/Fe alloy is reported to be approximately 1.2×10−6° C.−1 in the range of −400° C. to 0° C.−1 approximately 1.1×10−6° C.−1 in the range of −200° C. to 0° C., and approximately 0.5-1.1×10−6° C.−1 in the range of 25° C. to 93° C. In terms of the Celsius scale, the above CTE figures for 36Ni/Fe alloys may be compared with approximately 11-12×10−6° C.−1 for carbon steel, and approximately 22-24×10−6° C. for aluminum.
Early applications of 36 Ni/Fe alloys included surveying tapes and wires, grandfather clock pendulums, glass sealing wires, and applications in light bulbs and electronic vacuum tubes for radios. The rate of new applications for the 36Ni/Fe alloys accelerated throughout the 20th century. Indeed, even after over 100 years since its discovery, the uses found for 36Ni/Fe alloys continue to multiply, and the alloys have recently been applied in fields as diverse as semiconductors, aerospace, television, information technology, and cryogenics. In the 1980's and 1990's it was discovered that 36 Ni/Fe alloys are particularly useful as lining material for tanks and containers used to ship liquified natural gas since the alloys' thermal expansion properties limit cryogenic shrinkage. More recently, 36 Ni/Fe alloys have been applied in shadow masks in high-definition cathode ray (television) tubes, as structural components in precision laser and optical systems, in wave guide tubes, in microscopes, as elements of support systems for large-mirror telescopes, in various other instruments requiring mounted lenses, as tight dimensional tolerance molds for curing advanced composites at moderately high temperatures, in orbiting satellites, in lasers, and as components of ring laser gyroscopes.
As applications requiring highly dimensionally stable materials become increasingly sophisticated, the requirements for minimum thermal expansion and contraction characteristics have become more demanding. Accordingly, there is a need to develop alloys having CTE's that are lower than existing 36 Ni/Fe alloys. There is a further need to develop alloys containing iron and/or nickel, such as, for example, alloys within the 36Ni/Fe alloy family, having CTE's that are lower than existing 36 Ni/Fe alloys.